Elastomeric polymeric compositions are used to protect structures with forward facing surfaces, such as wings, rotor blades, propeller blades, fan blades, turbine blades, aircraft radome, and aircraft antennas. These structures can be severely damaged when used in their intended operational environments. The term “erosion damage” is a broad term encompassing damage caused by rain erosion, sand and dust erosion as well as impact damages caused by stone, gravel or foreign objects encountered typically in flight conditions.
Elastomeric erosion resistant coatings are supplied in the forms of tapes, sheets, molded boots and sprayable coatings. Currently available elastomeric polyurethane coatings used in erosion protection application are highly sand erosion resistant, demonstrating higher sand erosion resistance than metal. However, elastomeric polyurethane coatings have lower rain erosion resistance than metal, usually exhibiting rain erosion damage in the form of deep pits, cracks, craters, and holes. The size, shape and location of the damage sites vary depending on the nature of the damage. The size and shape can vary from crack lines as thin as hair lines, pits about 1 mini-meter or smaller in diameter, craters about 2 to 3 mini-meter in diameter, or irregularly shaped holes wider than 1 centimeter across. The damage sites can exist isolated and randomly distributed, or continuous across the forward facing surfaces.
When these erosion damages occur, it is extremely difficult to conduct repairs on the rain eroded polyurethane elastomers. The high sand erosion resistance makes it extremely difficult to remove the coatings by hand sanding. For helicopters, removal of the current types of erosion protection coating by mechanical or chemical means requires the removal of the rotor blades from the aircraft and typically removal by machine sanding or other techniques. The reapplication of the tape, molded boot and sprayable coatings in the field is very labor intensive and costly.
Another method to remove the damaged coating uses chemical strippers. This method also requires the removal of the rotor blades from the aircraft, as the open air will dry out the chemical stripper very quickly. Another problem is that chemical stripping introduces hazardous chemicals into the operation. In addition, typical erosion resistant coatings are used at a thickness equal or greater than 0.014″. It usually takes overnight soaking to soften the coatings so that they can be removed. There are also concerns that the stripper solution may swell and damage the composite structure under the erosion resistant coatings. For these reasons, it is not practical to do field repair with chemical stripper.
Possible methods that could be used to repair the erosion damage involve brushing on repair material and spraying on the repair materials. Neither of these methods is entirely satisfactory to fill in the cracks, holes of varying sizes and shapes on a curved surface, while still maintaining a smooth, aerodynamic surface at the end of the repair operations. The extra layers simply follow the irregular contours of the damaged surfaces interfering with aerodynamics of the airfoil. None of the methods employed to date have satisfactorily provided a method to field repair a rotor blade which has erosion damage.
It is an object of this invention to provide a method to efficiently fill in the pits, cracks, craters, and holes of varying sizes and shapes on a curved surface.
It is an object of this invention to provide a method to repair airfoil structures such as the rotor blades that can be accomplished in the field.
It is a further object of this invention to repair the rotor blade or other leading edge structure while the blades are still mounted on the aircraft or equipment,
It is an additional object to design an erosion protection system that can be removed and/or repaired in the field, without power tools or chemical strippers.
It is another objective of this invention to provide an erosion protection coating system for airfoils and a repairable resin system for airfoils with contrasting colors to allow early detection of erosion, impact and other damages, and to allow fast repair to lengthen the service life of the blades or structures.